Devices, Systems, and Methods for Visualizing an Occluded Vessel

ABSTRACT

Embodiments of the present disclosure are configured to visualize severe blockages in a vessel and, in particular, chronic total occlusions in blood vessels. In some particular embodiments, the devices, systems, and methods of the present disclosure are configured to visualize the blockage to facilitate safe crossing of the blockage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/706,015, filed Dec. 5, 2012, which claims priority to andthe benefit of U.S. Provisional Patent Application No. 61/568,498, filedDec. 8, 2011, each of which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present disclosure relates generally to the visualization of vesselsand, in particular, the visualization of vessels having a blockage orother restriction to the flow of fluid through the vessel. Aspects ofthe present disclosure are particularly suited for evaluation ofbiological vessels in some instances. For example, some particularembodiments of the present disclosure are specifically configured forthe visualizing and treating total occlusions of human blood vessels,such as a chronic total occlusion, an acute total occlusion, or a severestenosis.

BACKGROUND

Intravascular ultrasound (IVUS) imaging systems have been designed foruse by interventional cardiologists in the diagnosis and treatment ofcardiovascular and peripheral vascular disease. Such systems enhance theeffectiveness of the diagnosis and treatment by providing importantdiagnostic information that is not available from conventional x-rayangiography. This information includes the location, amount, andcomposition of arteriosclerotic plaque and enables physicians toidentify lesion characteristics, select an optimum course of treatment,position therapeutic devices and promptly assess the results oftreatment.

Such IVUS systems generally include an IVUS device having one or moreminiaturized transducers mounted on the distal portion of a catheter orguide wire to provide electronic signals to an external imaging system.The external imaging system produces an image of the lumen of the arteryor other cavity into which the catheter is inserted, the tissue of thevessel, and/or the tissue surrounding the vessel. Problems encounteredwith these systems include clearly visualizing the tissue around thecatheter, and identifying the precise location of the image with regardto known spatial references, such as angiographic references.

Before the development of less invasive approaches, the principal modeof treatment for occluded arteries was bypass surgery and, in the caseof occlusions in the coronary arteries, coronary artery bypass surgery.Coronary artery bypass surgery is a highly invasive procedure in whichthe chest cavity is opened to expose the heart to provide directsurgical access to the coronary arteries. The procedure also includesthe surgical removal of blood vessels from other locations in thepatient's body (e.g., the sapheneous vein) which then are graftedsurgically to the coronary arteries to bypass the occlusions. Therecuperative period is lengthy with considerable discomfort to thepatient.

The use of less invasive, catheter-based, intravascular techniques hasdeveloped for several decades and may be considered as the preferredmode of treatment for those patients amenable to such treatment.Typically, the intravascular procedures, such as angioplasty,atherectomy, and stenting require preliminary navigation of a guidewirethrough the patient's arteries to and through the occlusion. Thisguidewire, so placed, serves as a rail along which catheters can beadvanced directly to and withdrawn from the target site. Totalocclusions often cannot be treated with such minimally invasiveintravascular approaches because of the inability to advance a guidewirethrough the stenosis. Typically patients with such occlusions have beentreatable, if at all, by bypass surgery. Although in some instances,physicians may be able to force a guidewire through a total occlusion ifthe occluding material is relatively soft, attempts to force theguidewire through can present serious risks of perforating the artery.Arterial perforation can be life threatening.

The difficulties presented when trying to cross a total or near-totalocclusion are compounded by the typical manner in which the anatomy ofan occluded artery is diagnosed. Conventionally, such diagnosis involvesan angiographic procedure in which a radiopaque contrast liquid isinjected into the artery upstream of the occlusion and a radiographicimage is made. The resulting image is that of the compromised lumenwhich necessarily differs from the natural arterial lumen. Although withangiographic visualization techniques, the physician can determine thelocation of the occluded region and an indication of the degree ofobstruction, angiographic images do not provide a clear understanding ofwhere, in the occluded region, the natural boundaries of the vessel arelocated.

As used herein, the term “severe occlusion” or “severe obstruction” isintended to include total occlusions as well as those occlusions andstenoses that are so restrictive as to require preliminary formation ofa passage through the occlusion in order to receive additionalintravascular therapeutic devices. Such occlusions have various causesand occur in both the arterial or venous systems. Total or near totalocclusions occur in some instances as a consequence of the build-up ofplaque or thrombus, the latter being problematic in arteries as well asin the venous system. For example, deep veined thrombus and thromboticocclusion of vein grafts are serious conditions requiring treatment.

As noted above, recently techniques and systems have been developed tovisualize the anatomy of vascular occlusions by using intravascularultrasound (IVUS) imaging. IVUS techniques are catheter-based andprovide a real-time sectional image of the arterial lumen and thearterial wall. An IVUS catheter includes one or more ultrasoundtransducers at the distal portion of the catheter by which imagescontaining cross-sectional information of the artery under investigationcan be determined. The ultrasound transducer(s) are typically spacedfrom the distal tip of the catheter. In that regard, the catheterstypically include a distal tip formed of a radiopaque material such thatthe distal tip of the catheter is identifiable on fluoroscopy, x-ray,angiograph, or other similar imaging techniques. As a result of thedistal tip, the ultrasound transducer(s) may be anywhere from one tofive centimeters proximal of the distal tip of the catheter. Forexample, in each of the Atlantis SR Pro Imaging Catheter and iCrossCoronary Imaging Catheter available from Boston Scientific Corporation,the ultrasound transducer is positioned 2.1 cm proximal of a marker bandnear the distal tip such that the ultrasound transducer is approximately3 cm proximal of the distal tip of the device. Further, even in theEagleEye® Platinum RX Digital IVUS Catheter available from VolcanoCorporation, the transducer array is spaced from the distal tip by adistance of 1 cm. This spacing of the ultrasound transducer(s) from thedistal tip of the device is suitable for many vessel visualizationapplications and evaluations, but has limited effectiveness in thevisualization and evaluation of severe occlusions

Accordingly, there remains a need for improved devices, systems, andmethods for visualizing vessels having a severe blockage or otherrestriction to the flow of fluid through the vessel. In that regard,there remains a need for improved devices, systems, and methods forvisualizing the severe blockage to facilitate safely crossing theblockage.

SUMMARY

Embodiments of the present disclosure are configured to visualize ablockage in a vessel and, in particular, a severe blockage in a bloodvessel to facilitate crossing of that blockage. In some instances,devices particularly suited for visualizing a blockage are provided. Inthat regard, the devices include one or more imaging elements (such asultrasound, OCT, thermal, and/or other imaging modality) positionedadjacent the distal tip of the device. In some instances, the imagingelement(s) are positioned less than 5 mm, less than 4 mm, less than 3mm, less than 2 mm, less than 1 mm, and/or less than 0.5 mm from thedistal tip of the device. Further, in some implementations the device isa catheter that includes an inner lumen that is sized and shaped toreceive a guidewire. In that regard, in some embodiments the catheter isarranged as a rapid-exchange catheter having at least one opening incommunication with the central lumen for receiving the guidewire, theopening being positioned between the proximal and distal ends of thecatheter. In other embodiments, the catheter is an over-the-wirecatheter.

In other instances, methods of crossing a total occlusion of a vessel ofa patient are provided. The method includes introducing an imagingdevice into the vessel of the patient, advancing the imaging device to aposition immediately adjacent the total occlusion of the vessel suchthat a distal tip of the imaging device is in contact with the occlusionand one or more imaging elements (such as ultrasound, OCT, thermal,and/or other imaging modality) of the imaging device are positioned lessthan 5 mm, less than 4 mm, less than 3 mm, less than 2 mm, less than 1mm, and/or less than 0.5 mm from the occlusion. The method furtherincludes obtaining images of the vessel, including the occlusion, withthe imaging device positioned immediately adjacent the total occlusion.In some instances, the method further includes penetrating the totalocclusion based on the images obtained by the imaging device. In thatregard, in some instances an ablation guidewire or other occlusioncrossing device is advanced through a central lumen of the imagingdevice to the occlusion. In some instances, the occlusion is partiallypenetrated or crossed using the ablation guidewire (e.g., RF, laser,electric, plasma, etc.) or other occlusion crossing device (e.g.,needle, etc.), then the imaging device is advanced into the openingcreated by the partial penetration/crossing and again used to image thevessel, including the occlusion. This process can be repeated to safelyguide the ablation guidewire or other occlusion crossing device throughthe occlusion.

Additional aspects, features, and advantages of the present disclosurewill become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present disclosure will be describedwith reference to the accompanying drawings, of which:

FIG. 1 is a diagrammatic perspective view of an imaging device accordingto an embodiment of the present disclosure.

FIG. 2 is a diagrammatic perspective view of an imaging device accordingto another embodiment of the present disclosure.

FIG. 3 is a diagrammatic side view of a distal portion of an imagingdevice, such as the imaging devices shown in FIGS. 1 and 2, according toan embodiment of the present disclosure.

FIG. 4 is a close up diagrammatic side view of a distal tip of thedistal portion of the imaging device shown in FIG. 3.

FIG. 5 is a diagrammatic perspective view of an imaging system accordingto an embodiment of the present disclosure.

FIG. 6 is an isometric view of a side-looking or lateral imaging planeof an imaging device according to an embodiment of the presentdisclosure.

FIG. 7 is an isometric view of a forward-looking imaging plane of animaging device according to an embodiment of the present disclosure.

FIG. 8 is an isometric view of a forward-looking imaging plane of animaging device according to another embodiment of the presentdisclosure.

FIG. 9 is a diagrammatic, schematic view of an imaging system accordingto an embodiment of the present disclosure.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It is nevertheless understood that no limitation tothe scope of the disclosure is intended. Any alterations and furthermodifications to the described devices, systems, and methods, and anyfurther application of the principles of the present disclosure arefully contemplated and included within the present disclosure as wouldnormally occur to one skilled in the art to which the disclosurerelates. In particular, it is fully contemplated that the features,components, and/or steps described with respect to one embodiment may becombined with the features, components, and/or steps described withrespect to other embodiments of the present disclosure. For the sake ofbrevity, however, the numerous iterations of these combinations will notbe described separately.

Referring to FIG. 1, shown therein is an imaging device 100 according toan embodiment of the present disclosure. As shown, the imaging device100 comprises an elongate flexible body 102 having a proximal portion104 and a distal portion 106. The proximal portion 104 includes anadapter 108. In the illustrated embodiment, the adapter 108 is y-shapedwith extensions 110 and 112. In that regard, extension 110 generallyextends along the longitudinal axis of the body 102, while extension 112extends at an oblique angle with respect to the longitudinal axis of thebody. Generally, the extensions 110 and 112 provide access to the body102. In that regard, in the illustrated embodiment extension 110 isconfigured to receive a guidewire 114 that is sized and shaped to fitwithin a lumen that extends along the length of the body 102 from theproximal portion 104 to the distal portion 106 and defines an opening atthe distal end of the imaging device 100. As a result of thisarrangement, the imaging device 100 is understood to be what is commonlyreferred to as an over-the-wire catheter. In some embodiments, the lumenof the imaging device is centered about the central longitudinal axis ofthe body 102. In other embodiments, the lumen is offset with respect tothe central longitudinal axis of the body 102.

In the illustrated embodiment, extension 112 of adapter 108 isconfigured to receive communication lines (e.g., electrical, optical,and/or combinations thereof) that are coupled to imaging componentspositioned within the distal portion 106 of the imaging device 100. Inthat regard, a cable 116 containing one or more communication linesextends from extension 112 to a connector 118. The connector 118 isconfigured to interface the imaging device directly or indirectly withone or more of a patient interface module (“PIM”), a processor, acontroller, and/or combinations thereof. The particular type ofconnection depends on the type of imaging components implemented in theimaging device, but generally include one or more of an electricalconnection, an optical connection, and/or combinations thereof.

The distal portion 106 includes a plurality of markers 120. In thatregard, the markers 120 are visible using non-invasive imagingtechniques (e.g., fluoroscopy, x-ray, CT scan, etc.) to track thelocation of the distal portion 106 of the imaging device 100 within apatient. Accordingly, in some instances the markers 120 are radiopaquebands extending around the circumference of the body 102. Further, themarkers 120 are positioned at known, fixed distances from an imagingelement 122 and/or the distal end 124 of the imaging device 100 in someinstances. While the distal portion 106 has been illustrated anddescribed as having a plurality (two or more) of markers 120, in otherembodiments the distal portion 106 includes one marker or no markers.Further, in some embodiments, one or more components associated with theimaging element 122 can be utilized as a marker to provide a referenceof the position of the distal portion 106 of the imaging device 100.

The imaging element 122 may be any type of imaging element suitable forvisualizing a vessel and, in particular, a sever occlusion in a vessel.Accordingly, the imaging element may be an ultrasound transducer array(e.g., arrays having 16, 32, 64, or 128 elements are utilized in someembodiments), a single ultrasound transducer, one or more opticalcoherence tomography (“OCT”) elements (e.g., mirror, reflector, and/oroptical fiber), and/or combinations thereof. In that regard, in someembodiments the imaging device 100 is configured to be rotated (eithermanually by hand or by use of a motor or other rotary device) to obtainimages of the vessel.

Referring to FIG. 2, shown therein is an imaging device 200 according toanother embodiment of the present disclosure. As shown, the imagingdevice 200 comprises an elongate flexible body 202 having a proximalportion 204 and a distal portion 206. The proximal portion 204 includesa handle 208 for grasping by a user. In the illustrated embodiment, acable 216 extends from the handle 208 and includes one or morecommunication lines (e.g., electrical, optical, and/or combinationsthereof) that are coupled to imaging components positioned within thedistal portion 206 of the imaging device 200. In that regard, a cable216 containing one or more communication lines extends from handle 208to a connector 218. The connector 218 is configured to interface theimaging device directly or indirectly with one or more of a patientinterface module (“PIM”), a processor, a controller, and/or combinationsthereof. The particular type of connection depends on the type ofimaging components implemented in the imaging device, but generallyinclude one or more of an electrical connection, an optical connection,and/or combinations thereof.

The body 202 includes an opening 210 that is in communication with alumen that extends along the length of the body 202 from the opening 210to the distal portion 206 and defines an opening at the distal end ofthe imaging device 200. The opening 210 and the lumen it is incommunication with are configured to receive a guidewire. As a result ofthis arrangement, the imaging device 200 is understood to be what iscommonly referred to as a rapid exchange catheter. In some embodiments,the lumen of the imaging device is centered about the centrallongitudinal axis of the body 202. In other embodiments, the lumen isoffset with respect to the central longitudinal axis of the body 202.

The distal portion 206 includes a plurality of markers 220. In thatregard, the markers 220 are visible using non-invasive imagingtechniques (e.g., fluoroscopy, x-ray, CT scan, etc.) to track thelocation of the distal portion 206 of the imaging device 200 within apatient. Accordingly, in some instances the markers 220 are radiopaquebands extending around the circumference of the body 202. Further, themarkers 220 are positioned at known, fixed distances from an imagingelement 222 and/or the distal end 224 of the imaging device 200 in someinstances. While the distal portion 106 has been illustrated anddescribed as having a plurality (two or more) of markers 220, in otherembodiments the distal portion 206 includes one marker or no markers.Further, in some embodiments, one or more components associated with theimaging element 222 can be utilized as a marker to provide a referenceof the position of the distal portion 206 of the imaging device 200.

The imaging element 222 may be any type of imaging element suitable forvisualizing a vessel and, in particular, a sever occlusion in a vessel.Accordingly, the imaging element may be an ultrasound transducer array(e.g., arrays having 16, 32, 64, or 128 elements are utilized in someembodiments), a single ultrasound transducer, one or more opticalcoherence tomography (“OCT”) elements (e.g., mirror, reflector, and/oroptical fiber), and/or combinations thereof. In that regard, in someembodiments the imaging device 200 is configured to be rotated (eithermanually by hand or by use of a motor or other rotary device) to obtainimages of the vessel.

Referring now to FIGS. 3 and 4, shown therein is a distal portion 300 ofan imaging device according to an embodiment of the present disclosure.In that regard, the illustrated arrangement of the distal portion 300 issuitable for use in both over-the-wire catheters (e.g., imaging device100 of FIG. 1) and rapid exchange catheters (e.g., imaging device 200 ofFIG. 2). As shown, the distal portion 300 includes a main body 302 thecontains imaging components 304, which may include various electronic,optical, and/or electro-optical components necessary for the particularimaging modality utilized by the imaging device. In the illustratedembodiment, the distal portion 300 of the imaging device is configuredfor ultrasound imaging and includes an array 306 of ultrasoundtransducers arranged circumferentially about the distal portion 300 ofthe imaging device. In that regard, in some embodiments the transducerarray 306 and associated components 304 include features as disclosed inU.S. Pat. No. 5,857,974 to Eberle et al. that issued Jan. 12, 1999, U.S.Pat. No. 6,283,921 to Nix et al. that issued on Sep. 4, 2001, U.S. Pat.No. 6,080,109 to Baker et al. that issued on Jun. 27, 2000, U.S. Pat.No. 6,123,673 to Eberle et al. that issued on Sep. 26, 2000, U.S. Pat.No. 6,457,365 to Stephens et al. that issued on Oct. 1, 2002, U.S. Pat.No. 7,762,954 to Nix et al. that issued on Jul. 27, 2010, U.S. Pat. No.7,846,101 to Eberle et al. that issued on Dec. 7, 2010, and U.S. PatentApplication Publication No. 2004/0054287 that published on Mar. 18,2004, each of which is hereby incorporated by reference in its entirety.

As shown, the main body 302 of the distal portion 300 has a diameter orthickness 308. Generally, the diameter or thickness 308 of the distalportion 300 closely matches the diameter of the main body of the imagingdevice. In some instances, the diameter or thickness 308 of the distalportion 300 exactly matches the diameter of the main body of the imagingdevice. In other instances, the diameter or thickness 308 of the distalportion 300 is slightly larger or slight smaller than the diameter ofthe main body of the imaging device. In some instances, the diameter orthickness 308 is between about 0.5 mm and about 5 mm, with someparticular embodiments having a diameter or thickness of 2.73 mm (8.2French), 2.33 mm (7 French), 1.17 mm (3.5 French), 1.1 mm (3.3 French),1.0 mm (3 French), 0.97 mm (2.9 French), or otherwise.

The distal portion 300 also includes a tapered tip portion 310 thatextends distally from the main body 302 to the distal end 312. As shown,the tapered tip portion 310 transitions the distal portion 300 from thediameter or thickness 308 to a reduced diameter or thickness 314 at thedistal end 312. In some instances, the diameter or thickness 314 isbetween about 0.30 mm and about 2.5 mm, with some particular embodimentshaving a diameter or thickness of 0.30 mm (0.012″ or 0.9 French), 0.38mm (0.015″ or 1.14 French), 0.48 mm (0.019″ or 1.44 French), orotherwise. In that regard, the diameter or thickness 314 is determinedbased on the desired lumen size for the imaging device in someinstances. For example, as shown in FIGS. 3 and 4 a guidewire 114 isreceived within the lumen of the imaging device such that it extendsthrough an opening in the distal end 312 of the imaging device. In someparticular instances, the guidewire 114 has an outer diameter betweenabout 0.28 mm (0.011″ or 0.84 French) and about 0.46 mm (0.018″ or 1.38French) mm, with some embodiments having an outer diameter of 0.36 mm(0.014″ or 1.07 French). In other instances, the guidewire 114 has outerdiameter outside of this range, either larger or smaller. As the distalend 312 of the imaging device defines the opening that receives theguidewire, the diameter or thickness 314 is between 0.28 mm (0.011″ or0.84 French) and about 0.5 mm (0.020″ or 1.5 French) in someembodiments. In that regard, it is understood that the distal end 312 ofthe imaging device will necessarily have a slightly larger diameter orthickness than that of the guidewire 114 such that the guidewire can bereceived therein. However, in some instances the diameter or thickness314 of the distal end 312 of the imaging device is within 0.03 mm(0.001″ or 0.09 French) or less of the outer diameter of the guidewire.In other instances, the diameter or thickness 314 of the distal end 312of the imaging device is within 0.5 mm (0.020″ or 1.5 French) or less ofthe outer diameter of the guidewire.

As shown, the tapered tip portion 310 of the imaging device extendsproximal of the distal end 312 by a distance 316. In that regard, thedistance 316 is less than 5 mm in some embodiments. Further, thedistance 316 is less than 4 mm, less than 3 mm, less than 2 mm, lessthan 1 mm, and/or less than 0.5 mm from the distal end 312 of the devicein some instances. The distance 316 and the difference between thediameter or thickness 308 of the main body 302 and the diameter orthickness 314 at the distal end 312 determine the slope of the outersurface defined by the tapered tip portion 310. In that regard, in someembodiments the tapered tip portion 310 includes a constant taperbetween the diameter or thickness 308 of the main body 302 at theproximal end of the tapered tip portion and the diameter or thickness314 at the distal end 312 of the tapered tip portion. In otherinstances, the tapered tip portion 310 includes a variable taper betweenthe diameter or thickness 308 of the main body 302 at the proximal endof the tapered tip portion and the diameter or thickness 314 at thedistal end 312 of the tapered tip portion. For example, in someinstances the degree of taper decreases as the tapered tip portion 310extends distally towards the distal end 312.

Referring now to FIG. 5, there is shown a catheter 400 for intravascularuse, which may be similar to either of imaging devices 100 and 200discussed above. In that regard, this catheter has an elongated flexiblebody 402 with an axially extending lumen 404 through which a guide wire406, fluids, and/or various therapeutic devices or other instruments canbe passed. The present disclosure is not, however, limited to use withthe illustrated catheter arrangements, and it can be utilized with anysuitable catheter, guide wire, probe, etc. An ultrasonic imagingtransducer assembly 408 is provided at the distal portion 410 of thecatheter, with a connector 424 located at the proximal end of thecatheter. This transducer 408 comprises a plurality of transducerelements 412 that are preferably arranged in a cylindrical arraycentered about the longitudinal axis 414 of the catheter fortransmitting and receiving ultrasonic energy. The transducer elements412 are mounted on a cylindrical substrate 416 which, in the embodimentillustrated, consists of a flexible circuit material that has beenrolled into the form of a tube. A transducer backing material with theproper acoustical properties surrounds the transducer elements 412.

Each of the transducer elements 412 comprises an elongated body of PZTor other suitable piezoelectric material. The elements extendlongitudinally on the cylindrical substrate and parallel to the axis ofthe catheter. Each element has a rectangular cross-section, with agenerally flat surface at the distal end thereof. The transducerelements are piezoelectrically poled in one direction along their entirelength as highlighted. In some embodiments, a transversely extendingnotch of generally triangular cross-section is formed in each of thetransducer elements. The notch opens through the inner surface of thetransducer element and extends almost all the way through to the outersurface. Preferably, the notch has a vertical sidewall on the distalside and an inclined sidewall on the proximal side. The vertical wall isperpendicular to the longitudinal axis of the catheter, and the inclinedwall is inclined at an angle on the order of 60 degrees to the axis. Thenotch, which exists in all the array transducer elements, can be filledwith a stable non-conductive material. An example of a material that canbe used to fill notch is a non-conductive epoxy having low acousticimpedance. Although not the preferred material, conductive materialshaving low acoustic impedance may also be used to fill notch. If aconductive material is used as the notch filler, it could avoid havingto metalize the top portion to interconnect both portions of thetransducer elements as required if a nonconductive material is utilized.Conductive materials are not the preferred notch filler given that theyhave an affect on the E-fields generated by the transducer elements.

In the preferred embodiment, the transducer array provides for a forwardlooking elevation aperture for 10 mega Hertz (MHz) ultrasound transmitand receive, and a side looking elevation aperture for 20 MHz ultrasoundtransmit and receive. Other frequencies and/or frequency combinationscan be used depending on the particular design requirements or intendeduses for the imaging device. The transducer array is manufactured byelectrically and mechanically bonding a poled, metalized block of thepiezoelectric material to the flexible circuit substrate with thesubstrate in its unrolled or flat condition. The transducer blockexists, as a piezoelectrically poled state where the thickness-axispoling is generally uniform in distribution and in the same axisthroughout the entire block of material. If included, a notch is thenformed across the entire piezoelectric block, e.g. by cutting it with adicing saw. Each of the individual notches is filled with a materialsuch as plastic and a metallization is applied to the top of the notchto form a continuous transducer inner electrode with metallization. Theblock is then cut lengthwise to form the individual elements that areisolated from each other both electrically and mechanically, with kerfsformed between the elements. Cable wire attachment terminals areprovided on the substrate that allow microcables that are electricallyconnected to an external ultrasound system to connect with thetransducer assembly in order to control the transducers.

Integrated circuits are installed on the substrate and the substrate isthen rolled into its cylindrical shape, with the transducer elements onthe inner side of the cylinder. The sleeve of radiopaque material ismounted on the core, the core is positioned within the cylinder, and theacoustic absorbing material is introduced into the volume between thecore and the transducer elements. In the event that a radiopaque markeris not required for a particular application, it can be omitted. Thetransducer elements 412 can be operated to preferentially transmit andreceive ultrasonic energy in either a thickness extensional TE) mode(k₃₃ operation) or a length extensional (LE) mode (k₃₁ operation). Thefrequency of excitation for the TE mode is determined by the thicknessof the transducer elements in the radial direction, and the frequencyfor the LE mode is determined by the length of the body between distalend surface and the vertical wall of notch. The thickness TE mode isresonant at a frequency whose half wavelength in the piezoelectricmaterial is equal to the thickness of the element. And the LE mode isresonant at a frequency whose half wavelength in the piezoelectricmaterial is equal to the distance between the distal end and the notch.Each transducer element is capable of individually operating to transmitand receive ultrasound energy in either mode, with the selection of thedesired mode (i.e. “side”, or “forward”) being dependent upon; a) anelectronically selected frequency band of interest, b) a transducerdesign that spatially isolates the echo beam patterns between the twomodes, and c) image plane specific beam-forming weights and delays for aparticular desired image plane to reconstruct using synthetic aperturebeam-forming techniques, where echo timing incoherence between the“side” and “forward” beam patterns will help maintain modal isolation.

Referring now to FIGS. 6-8, shown therein are various imaging planesthat are utilized in some embodiments of the devices and methods of thepresent disclosure. In that regard, some of the ultrasonic imagingcatheters of the present disclosure are configured to be “side looking”devices that produce B-mode images in a plane that is perpendicular tothe longitudinal axis of the catheter and passes through the transducer.That plane can be referred to as the B-mode lateral plane and isillustrated in FIG. 6. Further, some of the ultrasonic imaging cathetersof the present disclosure are configured to be “forward looking” devicesthat produce a C-mode image plane that is perpendicular to the axis ofthe catheter and spaced distally from the transducer array, which isillustrated in FIG. 7. Further still, some of the ultrasonic imagingcatheters of the present disclosure are configured to be “forwardlooking” devices that produce a B-mode image in a plane that extends ina forward direction from the transducer and parallel to the axis of thecatheter. That imaging plane is referred to as the B-mode forward planeand is illustrated in FIG. 8. Forward viewing devices can beparticularly advantageous in some crossing severe occlusions as theyallow the physician to see aspects of the occlusion in front of thecatheter. Finally, some of the ultrasonic imaging catheters of thepresent disclosure are configured to transition between two or more ofthe imaging planes shown in FIGS. 6-8. The following discusses waysthese multiple modes of imaging can be implemented. It is understoodthat some embodiments of the present disclosure implement only a singleone of these imaging modes. Further, it is understood that any suitableoperating frequencies may be utilized for the different imaging modes,including frequencies between 10 MHz and 80 MHz, including withoutlimitation 10 MHz, 20 MHz, 40 MHz, and 80 MHz. The forward-lookingimaging modes described below utilize a 20 MHz operating frequency insome instances.

Multiple Modes of Imaging: Explanation of the Principals of Operation

A piezoelectric transducer, when properly excited, will perform atranslation of electrical energy to mechanical energy, and as well,mechanical to electrical. The effectiveness of these translationsdepends largely on the fundamental transduction efficiency of thetransducer assembly taken as a whole. The transducer is a threedimensional electromechanical device though, and as such is alwayscapable of some degree of electromechanical coupling in all possibleresonate modes, with one or several modes dominating. Generally animaging transducer design seeks to create a single dominate mode ofelectromechanical coupling, suppressing all other coupling modes as“spurious.” The common method used to accomplish a transducer designwith a single dominate mode of electromechanical coupling usually restsin the creation of a single, efficient mechanical coupling “port” to themedium outside of the transducer. The single port is created by mountingthe transducer such that the most efficient resonant mode of transduceroperation faces that mechanical coupling port, with all other modessuppressed by means of mechanical dispersion attained by transducerdimensional control and dampening materials.

In the design of the present disclosure, the transducer design utilizesthe fact that a transducer can be effective in two principalelectromechanical coupling modes, each mode using a different frequencyof operation, acoustic “port”, and electro-mechanical couplingefficiency. One port is the “side looking” port that is used in thecross-sectional view image (as shown in FIG. 6). The other port is the“end” or “forward looking” port of the array (as shown in FIGS. 7 and8).

The present disclosure allows the two electromechanical coupling modes(i.e. “side” and “forward”) to be always active, without any mechanicalswitching necessary to choose one mode exclusive of the other. Thisdesign also assures that echoes of any image target in the “sidelooking” plane (see FIG. 6) do not interfere with the targetreconstruction in the “forward looking” planes (see FIGS. 7 and 8), andreciprocally, image targets from the “forward looking” do not interferewith the target reconstruction in the “side looking” planes. Inaccordance with the disclosure, the design methods listed below are usedto maintain sufficient isolation between the two modes of operation.

A) Resonant and Spatial Isolation of the Two Modes

In some instances, the “side looking” port is designed for approximatelytwice the frequency of the “forward looking” port in accordance with thepreferred embodiment. The transducer dimensional design is such that the“high frequency and side looking” transducer port sensitivity to lowfrequency signals, and as well the “low frequency and forward looking”transducer port to high frequency signals, is very low.

Additionally, the transmit and receive acoustic “beam” directions of thetwo modes are at approximately right angles to each other and thisfeature offers an additional isolation with respect to image targetidentification. Also, as a means to further promote isolation betweenthe two modes of operation, and as well optimize a sparse array echocollection method, the echo collection process in “forward” beamreconstruction uses an intentional physical separation of transmittingand receiving transducer elements of preferably 10 elements or more inthe circular array annulus. This physical separation aids in preventing“spurious” transmit echoes from the “high frequency side looking” portfrom contaminating the receiving element listening to “forward only”echoes at the its lower frequency of operation.

B) Electrical Frequency Band Isolation of the Two Modes

As stated previously, the two modes of operation are operated at centerfrequencies that differ by about a factor of two. This design featureallows for additional isolation between the two modes through the use ofband pass filters in the host system that is processing the echo signalsreceived from the catheter. Additionally, if one or both of the twomodes is operated in a low fractional bandwidth design (i.e. <30%), thebandpass filters will be even more effective in the maintenance of veryhigh modal isolation.

C) Beam Formation Isolation Through Synthetic Aperture Reconstruction

Synthetic aperture beam reconstruction is used for all image modes. Thebeam formation process will preferentially focus only on image targetsthat are coherently imaged in a particular image plane. Thus, whileimage reconstruction is forming an image in, for example, the “sidelooking” plane, targets that may have contaminated the echoes from the“forward looking” planes will be generally incoherent and will besuppressed as a type of background noise. The reciprocal is also true:“side looking” echoes contaminants will be generally incoherent in“forward looking” imaging and will be suppressed through the process ofsynthetic aperture reconstruction.

A flexible digital image reconstruction system is required for thecreation of multiple image planes on demand. The preferred method ofassembling multiple image planes utilizes a synthetic aperturereconstruction approach. The “side looking” image shown in FIG. 1 can bereconstructed using sampled transducer element apertures as large as forexample 14 contiguous transducer elements in a 64 total transducerelement circular array. The transmit-receive echo collection foraperture reconstruction can be continuously shifted around the circulararray, sampling all transmit-receive cross-product terms to be used in aparticular aperture reconstruction. Within any 14-element aperture therecan be 105 independent transmit-receive echo cross products used toconstruct the image synthetically.

“Forward looking” images shown in FIGS. 7 and 8 can be reconstructedusing sampled apertures that consist of selected transducer elementsarranged on the annulus end of the circular array. For the 64 transducerelement example mentioned above, all elements may contribute to acomplete data set capture (this would consist of 64 by 32 independenttransmit-receive element cross-products) to form a “forward looking”image in either C-mode or B-mode. As an alternative to the complete dataset approach, a reduced number of independent transmit-receive elementcross-products are used to adequately formulate the image. Thetransmit-receive echo collection for aperture reconstruction can becontinuously shifted around the circular array, sampling alltransmit-receive element cross-products to be used in a particularaperture reconstruction.

Special signal processing may be advantageous, especially in the“forward looking” imaging modes that use a less efficient transducercoupling coefficient (k₃₁) and as well may suffer from additionaldiffraction loss not experienced in the “side looking” mode of syntheticaperture imaging. In forming a “forward looking” C-mode image plane asan example, a low noise bandwidth can be achieved by using a high numberof transmit pulses and a narrow bandpass echo filter in the processingsystem. Additionally, or as a preferred alternative, a matched filterimplementation from the use of correlation processing may be used toimprove the echo signal-to-noise ratio.

Standard Cross-Sectional B-Mode Operation

The advantage of this cross-sectional B-mode operation of the catheterimaging device is in its ability to see an image at great depth in theradial dimension from the catheter, and at high image resolution. Thisdepth of view can help aid the user of the catheter to position thedevice correctly prior to electronically switching to a “forwardviewing” mode of operation. Image targets moving quickly in a pathgenerally parallel to the long axis of the catheter can be detected anddisplayed as a colored region in this mode; this information can be usedto compare and confirm moving target information from the “forwardviewing” mode of operation of the catheter to enhance the usefulness ofthe imaging tool.

1. Transducer Operation

The transducer in this “primary” mode operates in the thicknessextensional (TE) resonance, utilizing the k₃₃ electro-mechanicalcoupling coefficient to describe the coupling efficiency. This“thickness resonance” refers to a quarter wave or half wave (dependingon the acoustic impedance of the transducer backing formulation)resonance in the transducer dimension that is in alignment with thepolarization direction of the transducer, and also the sensed or appliedelectric field. This TE mode utilizes a typically high frequencythickness resonance developed in the transducer short dimensionfollowing either electric field excitation to generate ultrasoundacoustic transmit echoes, or, in reception mode following acousticexcitation to generate an electric field in the transducer.

Array Stepping

The TE mode is used for generating a cross-sectional B-mode image. Thiscross-section image cuts through the array elements in an orthogonalplane to the long axis of the transducer elements. Echo informationgathered from sequential transducer element sampling around the arrayallows for the synthetically derived apertures of various sizes aroundthe array. For the creation of any synthetically derived aperture, acontiguous group of transducer elements in the array are sequentiallyused in a way to fully sample all the echo-independent transmit-receiveelement pairs from the aperture. This sequencing of elements to fullysample an aperture usually involves the transmission of echo informationfrom one or more contiguous elements in the aperture and the receptionof echo information on the same or other elements, proceeding until allthe echo independent transmit-receive pairs are collected.

Notch Effect

The small notch forming an acoustical discontinuity in the middle of thearray of some embodiments will have a minor, but insignificant effect onthe TE mode transmission or reception beam pattern for that element. Thesmall notch will be a non-active region for the TE mode resonance andtherefore contribute to a “hole” in the very near field beam pattern foreach element. The important beam characteristics however, such as themain lobe effective beam width and amplitude, will not be substantiallyaffected, and except for a very minor rise in the transducer elevationside lobes, reasonable beam characteristics will be preserved as if theentire length of the transducer element was uniformly active.

Modal Dispersion

The TE mode transducer operation will exist with other resonant modessimultaneously. The efficiency of electromechanical energy couplinghowever for each mode though depends on primarily these factors: a) thek coefficient that describes the energy efficiency of transduction for agiven resonance node, b) the acoustic coupling path to the desiredinsonification medium, and c) the echo transmission-reception signalbandwidth matching to the transducer resonance for that particular mode.Thus, for the creation of a “side looking” image, a transducer design iscreated to optimize the factors above for only the TE resonance, whilethe other resonant modes within a transducer are to be ignored throughthe design which suppresses the undesired resonances by minimizing theenergy coupling factors mentioned above.

Through this frequency dispersion of unwanted coupling, the desiredechoes transmitted and received from the “side looking” transducer portnecessary to create a B-mode image plane will be most efficientlycoupled through the TE resonance mode within any particular element.Therefore, the proposed transducer design which features a highefficiency TE mode coupling for desired echoes and frequency dispersionof the unwanted resonances and echoes, along with the other modalisolation reasons stated in an earlier section, constitutes a means forhigh quality TE echo energy transduction for only those desired in-planeechoes used in the creation of the B-mode cross-sectional image plane.

2. System Operation for the Standard Cross-Sectional B-Mode Imaging

The host ultrasound processing system shown in FIG. 9 controls theultrasound array 408 element selection and stepping process whereby asingle element 412 or multiple elements will transmit and the same orother elements will receive the return echo information. The elements inthe array that participate in a given aperture will be sampledsequentially so that all essential cross product transmit-receive termsneeded in the beam forming sum are obtained.

The host processing system or computer 914 and reconstruction controller918 will control the transmit pulse timing provided to widebandpulser/receiver 902, the use of any matched filter 910 via control line916 to perform echo pulse compression. The echo band pass filter (BPF)processing paths in the system are selected using control signal 906 toselect between either the 10 MHz 904 or 20 MHz 936 center frequency BPFpaths. The amplified and processed analog echo information is digitizedusing ADC 908 with enough bits to preserve the dynamic range of the echosignals, and passed to the beam-former processing section via signal912. The beam former section under the control of reconstructioncontroller 918 uses stored echo data from all the transmit-receiveelement pairs that exist in an aperture of interest. As the element echosampling continues sequentially around the circular array, all elementgroup apertures are “reconstructed” using well known synthetic aperturereconstruction techniques to form beam-formed vectors of weighted andsummed echo data that radially emanate from the catheter surface usingbeam-former memory array 922, devices 924 and summation unit 926. Memorycontrol signal 920 controls switch bank 924 which selects which memoryarray to store the incoming data.

The vector echo data is processed through envelope detection of the echodata and rejection of the RF carrier using vector processor 928. Finallya process of coordinate conversion is done to map the radial vectorlines of echo data to raster scan data using scan converter 930 forvideo display using display 932.

This processing system, through the host control, may also accomplishblood velocity detection by tracking the blood cells through theelevation length of the transducer beams. The tracking scheme involves amodification of the element echo sampling sequencing and the use of thebeam-former section of the host processing system. The blood velocityinformation may be displayed as a color on the video display; this bloodvelocity color information is superimposed on the image display to allowthe user to see simultaneous anatomical information and blood movementinformation.

Forward Looking Cross-Sectional C-Mode Operation

The advantage of this “forward looking” operation of the catheterimaging device is in its ability to see an image of objects in front ofthe catheter where possibly the catheter could not otherwise physicallytraverse. A “forward” C-mode plane produces a cross-sectional viewsimilar to the standard B-mode cross-sectional view, and so can offercomparable image interpretation for the user, and as well this forwardimage plane is made more useful because the user can see the presence ofimage targets at the center of the image, otherwise obscured in thestandard cross-sectional view by the catheter itself. This forward viewallows also the ideal acoustic beam positioning for the detection andcolor image display of Doppler echo signals from targets movinggenerally in parallel with the long axis of the catheter device.

1. Transducer Operation

The transducer in this “secondary” mode operates in the lengthextensional (LE) resonance, utilizing the k₃₁ electromechanical couplingcoefficient to describe the coupling efficiency. In this mode ofoperation, the poling direction of the transducer element and the sensedor applied electric field in the transducer are in alignment, but theacoustic resonance is at 90 degrees to the electric field and polingdirection. This “length resonance” refers fundamentally to a half waveresonance in the transducer element's length dimension that is at 90degrees with the polarization direction of the transducer. The LE modeof resonance, which is typically much lower in resonant frequency thanthe TE mode because the element length is normally much longer than thethickness dimension, always exists to some extent in a typicaltransducer array element, but is usually suppressed through a frequencydispersive design.

Some embodiments of the present disclosure utilize an abrupt physicaldiscontinuity (a notch) in the transducer element to allow a half waveLE resonance to manifest itself at a desired frequency, in the case ofthe preferred embodiment, at about one half the frequency of the TE moderesonance. A unique feature of this disclosure is a mechanically fixedtransducer design that allows two resonant modes to operate atreasonably high efficiencies, while the “selection” of a desired mode(i.e. “side”, or “forward”) is a function of a) an electronicallyselected frequency band of interest, b) a transducer design thatspatially isolates the echo beam patterns between the two modes, and c)image plane specific beam-forming weights and delays for a particulardesired image plane to reconstruct using synthetic aperture beam-formingtechniques, where echo timing incoherence between the “side” and“forward” beam patterns will help maintain modal isolation.

As discussed earlier, a resonant mode in a transducer design can be madeefficient in electromechanical energy coupling if at least the threefundamental factors effecting coupling merit are optimized, namely a)the k coefficient (in this case it is the k₃₁ electro-mechanicalcoupling coefficient) that describes the energy efficiency oftransduction for a given resonance node, b) the acoustic coupling pathto the desired insonification medium, and c) the echotransmission-reception signal bandwidth matching to the transducerresonance for that particular mode. The disclosure allows for reasonableoptimization of these factors for the LE mode of resonance, although theLE mode coupling efficiency is lower than that of the TE mode coupling.The k₃₁ coupling factor, used in describing LE mode efficiency, istypically one half that of k₃₃, the coupling factor that describes theTE mode efficiency.

The abrupt acoustical discontinuity in the transducer element is createdat a step in the assembly of the array. Following the attachment of thetransducer material to the flex circuit to create a mechanical bond andelectrical connection between the transducer block and the flex circuit,while the transducer material is still in block form, a dicing saw cutcan be made the entire length of the transducer material block, creatingthe notch. The notch depth should be deep enough in the transducermaterial to create an abrupt discontinuity in the distal portion of thetransducer material to allow for a high efficiency LE mode half waveresonance to exist in this end of the transducer element. The saw cutshould not be so deep as to sever the ground electrode trace on thetransducer block side bonded to the flex circuit. The cut should ideallyhave a taper on the proximal side to allow for acoustically emittedenergy to be reflected up into the backing material area and becomeabsorbed.

Generally, it is not desirable to have any acoustic coupling existbetween the LE modes of resonance in the distal and proximal transducerregions separated by the notch. The distal transducer region LE modehalf wave resonance will exist at 10 MHz in PZT (Motorola 3203HD) for alength of about 170 microns between the distal end of the transducerelement and the notch. The proximal transducer region LE mode resonancewill exist at a frequency considered out of band (approximately 6 MHz)in the two embodiments shown in FIGS. 5 and 7 so as to minimallyinterfere with the desired operating frequencies (in this case 10 MHz LEmode resonance in the distal region for “forward” acoustic propagation,and 20 MHz TE mode resonance in the entire active field length of thetransducer).

The desired acoustic energy coupling port of the distal transducer LEresonant mode region is at the distal end of the catheter array. Toprotect the end of the array and potentially act as an acoustic matchinglayer, an end cap made of polyurethane could be used, or alternatively,a uniform coating of adhesive material would suffice. The beam patternproduced by this acoustic port must be broad enough to insonify a largearea that covers intended extent of the image plane to be formed. Tothis end, the beam pattern must typically be at least 60 degrees wide asa “cone shaped” beam measured in the plane to be formed at thehalf-maximum intensity angles for 2-way (transmitted and received)echoes. The preferred design of the array has 64 or more elements, and atransducer sawing pitch equal to pi times the catheter array diameterdivided by the number of elements in the array. For an effective arraydiameter of 1.13 mm and 64 elements, the pitch is 0.055 mm. Using twoconsecutive array elements as a “single” effective LE mode acoustic portcan provide an adequate, uniform beam pattern that produces the required60-degree full-width half maximum (“FWHM”) figure of merit. The apertureof this “single” forward looking port is then approximately 0.080 mm by0.085 mm (where 0.085 mm is twice the pitch dimension minus the kerfwidth of 0.025 mm).

The transducer design may also include a version where no notch isneeded in the transducer block. In this case, the driven electrode canexist all along one surface of the transducer element, and the ground orreference electrode can exist all along the opposite side of theelement. The long axis length of the transducer will resonate at a halfwavelength in LE mode, and the thickness dimension will allow theproduction of a TE mode resonance in that thickness dimension. In orderfor this design to operate though, the LE and TE mode resonantfrequencies will be quite different in order to maintain the proper TEmode elevation beam focus. As an example, in maintaining the length ofthe active region of the element for an adequately narrow 20 MHz TE modeelevation beam width at 3 mm radially distant from the catheter, theelement length should be approximately 0.5 mm long. The resulting halfwave resonance frequency in LE mode then will be about 3 MHz. Thisdesign can be used for dual-mode imaging, but will not offer thefocusing benefits that 10 MHz imaging can offer for the forward lookingimage planes. Other designs are possible, where the forward frequency ismaintained near 10 MHz, but the required frequency for the side-lookingmode will rise dramatically, and although this can be useful in itself,will complicate the design by requiring a concomitant increase in thenumber of elements and/or a reduction in the array element pitchdimension.

2. System Operation

The host processing system will control the array element selection andstepping process whereby one element, a two element pair, or othermultiple elements in combination, will transmit and the same or otherelements will receive the return echo information. The intended arrayoperational mode is the LE resonant mode to send and receive echoinformation in a forward direction from the end of the catheter array.As stated earlier, the LE mode echoes produced may be isolated from theTE mode echoes through primarily frequency band limitations (both bytransducer structural design and by electrical band selection filters),and through the beam-forming reconstruction process itself as a kind ofecho selection filter.

To produce an image of the best possible in-plane resolution whileoperating in the forward-looking cross-sectional C-mode, the entirearray diameter will be used as the maximum aperture dimension. Thismeans that, in general, element echo sampling will take place at elementlocations throughout the whole array in preferably a sparse samplingmode of operation to gather the necessary minimum number ofcross-product echoes needed to create image resolution of high qualityeverywhere in the reconstructed plane.

By using transmit-receive echo contributions collected from elementsthroughout the whole catheter array, using either a “complete data set”(e.g. 64×32), or a sparse sampling (e.g. less than 64×32) of elements,the FWHM main beam resolution will be close to the 20 MHz resolution ofthe “side looking” cross-sectional image. This is due to the fact thatalthough the “forward looking” echo frequency is about one half as muchas the “side looking” frequency, the usable aperture for the forwardlooking mode is about 1.6 times that of the largest side lookingaperture (i.e. the largest side looking aperture is about 0.7 mm, andthe forward aperture is about 1.15 mm). For a 10 MHz forward lookingdesign, the FWHM main lobe resolution in an image plane reconstructed ata depth of 3 mm will be approximately 0.39 mm, and 0.65 mm resolution at5 mm distance.

Due to the limitation of beam diffraction available in the design using10 MHz as the echo frequency for “forward looking”, the C-mode imagediameter that can be reconstructed and displayed with a high level ofresolution from echo contributions throughout the whole array will berelated to the distance between the reconstructed C-mode image plane andthe distal end of the catheter. At 3 mm from the end of the catheter,the C-mode image diameter will be about 2.3 mm, at 5 mm distance theimage diameter will be 4.6 mm, and at 7 mm distance the image diameterwill be 6.9 mm.

The host processing system, in addition to the control of the transducerelement selection and stepping around the array, will control thetransmit pulse timing, the use of any matched filter to perform echopulse compression, and the echo band pass filter processing path in thesystem. The amplified and processed analog echo information is digitizedwith enough bits to preserve the dynamic range of the echo signals, andpassed to the beam-former processing section. The beam former sectionuses stored echo data from the sparse array sampling (or alternativelythe whole complete array echo data set of 64.times.32 oftransmit-receive element pairs) that exist in an aperture of interest.As the element echo sampling continues sequentially around the circulararray 1108 as shown in FIGS. 10 and 11, a number of “full trips” aroundthe array will have been made to collect a sufficient number of echocross-products (up to 105 in the preferred sparse sampling method) toallow the reconstruction of one image vector line 1102. As cross-productsampling continues around the array, the “older” echo cross-productcollections are replaced with new samples and the next image vector isformed. This process repeats through an angular rotation to create newimage vectors while sampling their element cross-product contributorsaround the array. In the same manner as described in the processing ofthe “side looking” image, the vector echo data is processed throughenvelope detection of the echo data and rejection of the RF carrier.Finally a process of coordinate conversion is done to map the radialvector lines of echo data to raster scan data for video display.

This processing system, through the host control, may also accomplish“forward looking” target (such as blood cells) velocity detection byeither correlation-tracking the targets along the “forward looking”direction (with processing as earlier discussed with the “side looking”approach), or by standard Doppler processing of echo frequency shiftsthat correspond to target movement in directions parallel with the“forward looking” echo paths. The target (e.g. blood) velocityinformation may be displayed as a color on the video display; thisvelocity color information is superimposed on the image display to allowthe user to see simultaneous anatomical information and target movementinformation.

Forward Looking Sagittal-Sectional B-Mode Operation

The advantage of the “forward looking” operation of the catheter imagingdevice is in its ability to see an image of objects in front of thecatheter where possibly the catheter could not otherwise physicallytraverse. “Forward” B-mode plane imaging produces a cross-sectionalplanar “sector” view (see FIG. 8) that can exist in any plane parallelto the catheter central axis and distal to the end of the catheterarray. This imaging mode may be used, in addition, to produce image“sector” views that are tilted slightly out of plane (see FIG. 8), andas well, may produce individual or sets of image “sectors” rotatedgenerally about the catheter axis to allow the user to see a multitudeof forward image slices in a format that shows clearly themultidimensional aspects of the forward target region of interest. Thisforward B-mode imaging (as with C-mode plane imaging) utilizes the idealacoustic beam positioning for the detection and color image display ofDoppler echo signals from targets moving generally in parallel with thelong axis of the catheter device.

1. Transducer Operation

The transducer operation in creating the “forward looking” B-mode imageformat is virtually the same as discussed earlier for creating the“forward looking” C-mode image. The transducer in this “secondary” modeoperates in the length extensional (LE) resonance, utilizing the k₃₁electromechanical coupling coefficient to describe the couplingefficiency. As with the C-mode image creation, the number of elementsused at any time to form a wide beam pointing in the “forward” directionare selected to produce a required 60 degree FWHM beam widthperformance; the modal isolation techniques mentioned earlier againstthe higher frequency TE resonances are valid as well for this forwardB-mode imaging method.

However, where it is merely preferred to operate the “forward” C-modeimaging with high bandwidth echo signals (low bandwidth echo signals canalso be used, but with some minor loss in image resolution), it is arequirement in the “forward” B-mode imaging that only high bandwidthecho signals (echo fractional bandwidth greater than 30%) be used topreserve the “axial” resolution in the “forward” B-mode image. Thelateral resolution in the “forward” B-mode image is determined (as theC-mode image plane resolution) by the aperture (diameter of the array)used for the image reconstruction. The lateral resolution performancewill be as stated earlier (i.e. from the description of the C-modeimaging case) for various depths from the catheter distal end.

2. System Operation

The system operation in creating the “forward looking” B-mode imageformat is largely the same as discussed earlier for creating the“forward looking” C-mode image, with the difference being in the use ofthe echo signals collected in the beam-forming process to create, ratherthan a C-mode image plane, a “forward” sagittal B-mode image in a planethat effectively cuts through the center of the circular array at thedistal end of the catheter.

The host processing system, as shown in FIG. 9, will control the arrayelement selection and stepping process whereby one element, a twoelement pair, or other multiple elements in combination, will transmitand the same or other elements will receive the return echo information.The intended array operational mode is the LE resonant mode to send andreceive echo information in a forward direction from the end of thecatheter array. As stated earlier, the LE mode echoes produced may beisolated from the TE mode echoes through primarily frequency bandlimitations (both by transducer structural design and by electrical bandselection filters), and through the beam-forming reconstruction processitself as a kind of echo selection filter.

To produce an image of the best possible in-plane resolution whileoperating in the “forward looking” sagittal B-mode, the entire arraydiameter will be used as the maximum aperture dimension. This meansthat, in general, element echo sampling will take place at elementlocations throughout the whole array in preferably a sparse samplingmode of operation to gather the necessary minimum number ofcross-product echoes needed to create image resolution of high qualityeverywhere in the reconstructed plane. By using transmit-receive echocontributions collected from elements throughout the whole catheterarray, using either a “complete data set” (e.g. 64×32), or a sparsesampling (e.g. less than 64×32) of elements, the FWHM main beam lateralresolution in the B-mode plane will be close to the 20 MHz resolution ofthe “side looking” cross-sectional image. Similarly, as stated earlierfor the C-mode image case, in the creation of the B-mode image using a10 MHz forward looking design, the FWHM main lobe lateral resolution inthe image plane reconstructed at a depth of 3 mm will be approximately0.39 mm, and 0.65 mm resolution at 5 mm distance.

Due to the limitation of beam diffraction available in the design using10 MHz as the echo frequency for “forward looking”, the B-mode sectorimage width that can be reconstructed and displayed with a high level ofresolution from echo contributions throughout the whole array will berelated to the distance between the reconstructed B-mode target depth inthe image sector and the distal end of the catheter. At 3 mm from theend of the catheter, the B-mode image sector width will be about 2.3 mm,at 5 mm distance the image sector width will be 4.6 mm, and at 7 mmdistance the image sector width will be 6.9 mm.

The host processing system, in addition to the control of the transducerelement selection and stepping around the array, will control thetransmit pulse timing, the use of any matched filter to perform echopulse compression, and the echo band pass filter processing path in thesystem. The amplified and processed analog echo information is digitizedwith enough bits to preserve the dynamic range of the echo signals, andpassed to the beam-former processing section. The beam former sectionuses stored echo data from the sparse array sampling (or alternativelythe whole complete array echo data-set of 64.times.32 oftransmit-receive element pairs) that exist in an aperture of interest.As the element echo sampling continues sequentially around the circulararray, a number of “full trips” around the array will have been made tocollect a sufficient number of echo cross-products (up to 105 in thepreferred sparse sampling method) to allow the reconstruction of oneimage vector line. As cross-product sampling continues around the array,the “older” echo cross-product collections are replaced with new samplesand the next image vector is formed. This process repeats through anangular rotation in the array to create new image vectors while samplingtheir element cross-product contributors around the array.

The method used for the creation of a single “forward looking” sagittalB-mode image plane may be expanded to create multiple rotated sagittalplanes around an axis either congruent with the catheter central axis,or itself slightly tilted off the catheter central axis. If enoughrotated planes are collected, the beam-forming system could then possessa capability to construct and display arbitrary oblique “slices” throughthis multidimensional volume, with B-mode or C-mode visualization ineither a 2-D sector format, a 2-D circular format, or, othermultidimensional formats. The echo data volume may also be off-loaded toa conventional 3-D graphics engine that could create the desired imageformat and feature rendering that would enable improved visualization.In the same manner as described in the processing of the “forwardlooking” C-mode image, the vector echo data is processed throughenvelope detection of the echo data and rejection of the RF carrier.Finally a process of coordinate conversion is done to map the radialvector lines of echo data to a video sector-format display of the“forward looking” B-mode image.

This processing system, through the host control, may also accomplish“forward looking” target (such as blood cells) velocity detection byeither correlation-tracking the targets along the “forward looking”direction (with processing as earlier discussed with the “side looking”approach), or by standard Doppler processing of echo frequency shiftsthat correspond to target movement in directions parallel with the“forward looking” echo paths in the “forward looking” B-mode plane. Thetarget (e.g. blood) velocity information may be displayed as a color onthe video display; this velocity color information is superimposed onthe image display to allow the user to see simultaneous anatomicalinformation and target movement information.

The disclosure has a number of important features and advantages. Itprovides an ultrasonic imaging transducer and method that can be usedfor imaging tissue in multiple planes without any moving parts. It canoperate in both forward and side imaging modes, and it permits imagingto be done while procedures are being carried out. Thus, for example, itcan operate in a forward looking C-mode, while at the same time atherapeutic device such as a laser fiber-bundle can be used to treattissue (e.g. an uncrossable arterial occlusion) ahead of the cathetertip either by tissue ablation, or, tissue photochemotherapy. The laserpulses may be timed with the ultrasound transmit-receive process so thatthe high frequency laser induced tissue reverberations can be seen inthe ultrasound image plane simultaneously. In this way the disclosurecan dynamically guide the operator's vision during a microsurgicalprocedure.

In some instances, the present disclosure is directed to a method ofcrossing a severe occlusion of a vessel of a patient. In that regard,the method includes introducing a flexible, elongate imaging device intothe vessel of the patient, advancing the imaging device to a positionimmediately adjacent the severe occlusion of the vessel such that atapered distal tip of the imaging device is in contact with theocclusion and such that at least one imaging element of the imagingdevice is spaced from the occlusion by a distance less than 5 mm, lessthan 3 mm, or less than 1 mm; and obtaining images of the vessel,including the occlusion, with the imaging device positioned immediatelyadjacent the severe occlusion. In some instances, the imaging device isan ultrasound device and the at least one imaging element is anultrasound transducer. In other instances, the imaging device is anoptical coherence tomography device and the at least one imaging elementis an optical fiber or a reflector. Further, in some embodimentsflexible, elongate imaging device is a catheter, such as arapid-exchange catheter or an over-the-wire catheter. The method alsoincludes penetrating the severe occlusion based on the images obtainedby the imaging device. In that regard, penetrating the severe occlusionincludes advancing an occlusion crossing device through a central lumenof the catheter to the occlusion. The occlusion crossing device may beone or more of an ablation device and a puncture device. In someinstances, penetrating the severe occlusion comprises partially crossingthe severe occlusion such that a recess is created in the occlusion, andthe method further includes advancing the imaging device into the recesscreated by the partial crossing; obtaining images of the vessel,including the partially crossed occlusion, with the imaging devicepositioned within the recess; and further penetrating the severeocclusion based on the images obtained by the imaging device whilepositioned within the recess. This process can be repeated until theocclusion has been completely crossed. Further, in some instances, afterthe occlusion has been crossed a balloon or other expansion mechanismmay be introduced into the opening created through the occlusion andused to further expand the opening. In some instances, the balloon orother expansion mechanism is attached to or formed as part of theimaging device.

In some embodiments, an imaging device for use in imaging a severeocclusion of a vessel of a patient is provided. The device includes anflexible elongate body having proximal portion and a distal portion, theflexible elongate body having a constant diameter along a majority ofits length between the proximal and distal portions, the distal portiondefining a distal tip that tapers from the constant diameter of theflexible elongate body to a smaller diameter as the distal tip extendsdistally along a longitudinal axis of the flexible elongate body,wherein the tapered portion of the distal tip has a length less than 5mm as measured along the longitudinal axis of the flexible elongatebody, and wherein at least the distal portion of the flexible elongatebody includes a lumen extending along its length; and at least oneimaging element secured to the distal portion of the flexible elongatebody proximal of the tapered portion of the distal tip such that the atleast one imaging element is spaced from a distal end of the flexibleelongate body by a distance of 5 mm or less. In some embodiments, theimaging device is an ultrasound device and the at least one imagingelement is an ultrasound transducer, such as single ultrasoundtransducer or an array of ultrasound transducer elements. In otherembodiments, the imaging device is an optical coherence tomographydevice and the at least one imaging element is an optical fiber or areflector. In some instances, the lumen is in communication with anopening in a sidewall of the flexible elongate body such that theimaging device is configured as a rapid-exchange catheter. In someinstances, the lumen extends along a full length of the flexibleelongate body such that the imaging device is configured as anover-the-wire catheter.

Aspects of the present disclosure can also be used in a biopsy oratherectomy procedure to allow the operator to perform a tissueidentification prior to tissue excision; the advantage being that thecatheter or biopsy probe device can be pointing in the general directionof the target tissue and thus aid significantly in the stereotaxicorientation necessary to excise the proper tissue sample. The disclosurecan also be used for the proper positioning of a radiotherapy core wirein the treatment of target tissue that exists well beyond the distalextent of the catheter.

Persons skilled in the art will recognize that the apparatus, systems,and methods described above can be modified in various ways.Accordingly, persons of ordinary skill in the art will appreciate thatthe embodiments encompassed by the present disclosure are not limited tothe particular exemplary embodiments described above. In that regard,although illustrative embodiments have been shown and described, a widerange of modification, change, and substitution is contemplated in theforegoing disclosure. It is understood that such variations may be madeto the foregoing without departing from the scope of the presentdisclosure. Accordingly, it is appropriate that the appended claims beconstrued broadly and in a manner consistent with the presentdisclosure.

What is claimed is:
 1. An imaging device for use in imaging a severeocclusion of a vessel of a patient, the device comprising: an flexibleelongate body having proximal portion and a distal portion, the distalportion defining a distal tip having a tapered portion that tapers to asmaller diameter as the distal tip extends distally along a longitudinalaxis of the flexible elongate body, wherein the tapered portion of thedistal tip has a length less than 5 mm as measured along thelongitudinal axis of the flexible elongate body, and wherein a lumenextends through at least the tapered portion of the distal tip to definean opening in the distal tip; and an array of imaging elements coupledto the distal portion of the flexible elongate body in an annularconfiguration proximal of the tapered portion of the distal tip suchthat the array of imaging elements is disposed adjacent the taperedportion of the distal tip and positioned 5 mm or less from a distal mostend of the distal tip.
 2. The device of claim 1, wherein the array ofimaging elements includes at least 16 imaging elements.
 3. The device ofclaim 1, wherein the array of imaging elements includes at least 64imaging elements.
 4. The device of claim 1, wherein the array of imagingelements is configured for side-looking imaging.
 5. The device of claim1, wherein the array of imaging elements is configured forforward-looking imaging.
 6. The device of claim 1, wherein the imagingdevice is an ultrasound device and wherein the imaging elements includeultrasound transducers.
 7. The device of claim 1, wherein the imagingdevice is an optical coherence tomography device and wherein the imagingelements include an optical fiber or a reflector.
 8. The device of claim1, wherein the lumen is in communication with an opening in a sidewallof the flexible elongate body such that the imaging device is configuredas a rapid-exchange catheter.
 9. The device of claim 1, wherein thelumen extends along a full length of the flexible elongate body suchthat the imaging device is configured as an over-the-wire catheter. 10.The device of claim 1, wherein the lumen is coaxial with at least thedistal portion of the flexible elongate body.
 11. The device of claim 1,wherein the lumen is offset with respect to the longitudinal axis of theflexible elongate body.
 12. The device of claim 1, wherein the lumen issized and shaped to receive a guidewire.
 13. The device of claim 12,wherein the lumen has a diameter configured to receive a guidewirehaving an outer diameter of 0.014″ or 0.018″.
 14. The device of claim 1,wherein the tapered portion has a constant taper.
 15. The device ofclaim 1, wherein the tapered portion has a variable taper.
 16. Thedevice of claim 1, wherein the distal tip has a length less than 3 mm asmeasured along the longitudinal axis of the flexible elongate body andwherein the array of imaging elements is disposed adjacent the taperedportion of the distal tip and positioned 3 mm or less from a distal mostend of the distal tip.
 16. The device of claim 1, wherein the distalportion includes at least one marker visible using non-invasive imagingtechniques.
 17. The device of claim 16, wherein the at least one markeris a radiopaque marker.
 18. The device of claim 16, wherein the distalportion includes at least two markers visible using non-invasive imagingtechniques.